1. Field of the Invention
The invention relates to directive antennas in general and, more particularly, to radar system antenna feed arrangements for generating at least two angularly separated beams in space from a linear array of antenna elements.
2. Description of the Prior Art
In the prior art, various scanning and/or tracking radar equipments require the generation of at least one pair of angularly separated beams which can be electronically (inertialessly) switched without modification of the excitation frequency of the antenna system.
Other well known prior art radar systems employ the so-called mono-pulse technique in which a pair of angularly close-spaced beams are required, associated circuitry being operative to produce the so-called sum and difference patterns employed in mono-pulse.
The concept of a dual-mode slotted-waveguide array where the propagation coefficients of the two modes can be independently adjusted to produce separate angularly separated beams in space is known. Such a system is described in U.S. Pat. No. 4,164,742 assigned to the assignee of the present application. In that reference, the mechanism employed to achieve the difference phase velocities for the two modes relies upon selection of the amount of energy coupled through a common wall between parallel waveguides. Accordingly, two simultaneous beams can be independently located, usually with relatively small angular separation, at a single excitation frequency and in the plane parallel to the array length. The array itself in that situation is preferably comprised of slots in a narrow wall of one of the two coupled parallel waveguides.
The concept disclosed in that reference U.S. patent is further discussed and applied to the mono-pulse situation in a paper entitled "Dual-Beam Waveguide Slot Array for Monopulse Application". That paper by G. A. Hockham and R. I. Wolfson, appeared in the International IEEE AP-S Symposium Digest, pages 688-691, June 1979.
Basically, any device employing a waveguide encounters the so-called "cut-off" frequency problem and is inherently dispersive. That is, looking at the waveguide itself as a piece of transmission line, it will be realized that the phase delays encountered along the transmission line are functions of frequency, this because waveguide TE modes are dispersive and subject to cut-off. The result of this phenomenon is that, in the prior art dual-beam waveguide device, the angular separation between the two beams does not remain constant for every operating frequency encountered.
Since the invention concerns non-dispersive, dual-mode transmission line elements, it may theoretically be implemented in coaxial line, stripline, microstrip, slab line or the like, however, the implementation in stripline hereinafter particularly described is considered the preferred embodiment. For further background in connection with suitable instrumentation of strip transmission line (stripline) for purposes of constructing the present invention, the prior art of knowledge to the practitioner in this art is relied upon. Two additional technical papers are particularly pertinent in respect to the technical background necessary for an understanding of and for practice of the invention. These are the paper entitled "Shielded Coupled-Strip Transmission Line", by S. B. Cohn, IEEE Transactions, Vol. MTT-3, pages 29-38, October 1955; and the paper "Problems in Strip Transmission Lines", also by S. B. Cohn, appearing in IEEE Transactions, Vol. MTT-3, pages 119-126, March 1955.
The details of the stripline implementation according to the invention are presented hereinafter.